According to Life & Style, the pair recently attended the anniversary of TAO restaurant and nightclub in Las Vegas and are said to be "super into each other".
"Sarah was there as Jack's date," a source told the publication.
"He took her to Vegas to celebrate his birthday with him."
The insider added: "TAO presented Jack with a birthday cake, and Sarah was by his side when he blew out the candles.
"They're very much on, full speed ahead. They seem super into each other! Both seem very happy."